1. Field of the Invention
The present invention relates to and air gap prism for use in double-plate image sensing devices, and method for producing same.
2. Description of the Related Art
Heretofore, image sensing devices which incorporate a plurality of image sensing elements and an optical prism have been proposed as image sensing devices with image sensing elements used in video cameras or digital cameras or the like for the purpose of high resolution. One example of am effective optical prism is an air gap prism comprising an assembly of a plurality of glass members and provided with air gaps between said glass members.
Metal foil (e.g., Japanese Examined Patent Application No. HEI 6-60962), and resin film have been used as the material of members supporting the air gap.
There are methods of forming beforehand an air gap supporting area on a surface corresponding to a glass member instead of using a thin plate-like air gap supporting member.
Examples of such methods of forming an air gap supporting area include methods wherein resin or the like is applied like printing and hardened (e.g., Japanese Laid-Open Patent Application No. HEI 2-19093), and methods wherein a metal or dielectric member is applied by vacuum deposition.
Astigmatic difference, a type of astigmatism, occurs because the air gap of an air gap prism is provided at an inclination relative to the optical axis of the image forming optical system, and is a factor adversely affecting image forming characteristics. Since the magnitude of this astigmatic difference is proportional to the thickness of the air gap, it is desirable that the air gap is made thin within a range which does not produce interference of the light rays, and theoretically suitable values are less than 10 microns. It is important that the air gaps are parallel since astigmatic difference is markedly larger and causes adverse affects such as color separation when the surfaces forming the air gap are not parallel.
In high precision image sensing devices of recent years, there has been demand for high precision optical systems via high density pixel arrangements of image sensing elements. In optical prisms, there has been demand for producing thinner air gaps with higher precision.
In the case of air gap supporting areas formed in direct contact with glass members, the air gap is controlled by the support member thickness during said formation. Although the position and thickness of the formed air gap supporting members are accurate in methods for vacuum deposition of metals and dielectrics, the cost increases over the time required for a vacuum deposition process to obtain a desired thickness.
On the other hand, although methods of applying resin and the like are economical in terms of time and materials and the process itself is simple, a large number of man hours are required to adjust conditions for the resin application. Furthermore, the thin plate-like air gap supporting members have individual differences relative to uniformity of thickness and are subject to deterioration, making them unsuitable for high precision image sensing devices demanding high accuracy in the degree of parallelism and dimensions of the air gaps.
Thin plate-like air gap supporting members have many long, narrow, band-like sections which, during assembly, are difficult to accurately place at desired positions on a surface corresponding to a glass member.
Metal foil air gap supporting members are expensive due to the various processes involved to eliminate sections protruding from the prism during formation as well as suppressing surface reflection. In contrast, resin film provides easy formation and excellent precision at low cost, but are readily susceptible to creasing, bending, and wrinkling at thicknesses of 10 micron and less, such that assembly of the prism is extremely difficult.